Double-hydraulic actuator

ABSTRACT

Control for a tandem connected, redundant servo motor system. The tandem connected servo motors are each provided with a suitable control valve and independent pressure fluid supplies therethrough. A single control linkage is provided for both control valves and said linkage is connected to each valve by mechanically over-rideable means so that if one valve sticks the other can still be operated. Means are provided normally holding energizing pressure on a selected first motor and off the second motor, said means being responsive to an over-riding of said over-rideable means to reverse the pressure conditions on the motors. By making the normal conditions of said means partially dependent upon the existence of the main supply pressure, same can be caused to turn on the auxiliary supply pressure upon failure of the main supply pressure.

[ Apr. 22, 1975 1 DOUBLE-HYDRAULIC ACTUATOR [75] Inventor:

[73] Assignee: Messerschmitt-Biilkow-Blohm GmbI-I, Munich, Germany [22]Filed: Feb. 22, 1972 [21] Appl. No.: 227,889

I-Ians Zech, Munich, Germany [30] Foreign Application Priority Data Feb.23, 1971 Germany 2108545 [52] U.S.Cl. 91/411; 91/411 A;9l/412; 91/413[51] Int. Cl. Fl5b 11/16; F15b 13/06 [58] Field of Search 91/41 1, 411A, 413, 363 A, 91/412, 384; 137/637.1

FOREIGN PATENTSOR APPLICATIONS United Kingdom 91/412 PrimaryExaminer-lrwin C. Cohen Attorney, Agent, or Firm-Woodhams, Blanchard andFlynn [57] ABSTRACT Control for a tandem connected, redundant servomotor system. The tandem connected servo motors are each provided with asuitable control valve and independent pressure fluid suppliestherethrough. A single control linkage is provided for both controlvalves and said linkage is connected to each valve by mechanicallyover-rideable means so that if one valve sticks the other can still beoperated. Means are provided normally holding energizing pressure on aselected first motor and off the second motor, said means beingresponsive to an over-riding of said overrideable means to reverse thepressure conditions on the motors. By making the normal conditions ofsaid means partially dependent upon the existence of the main supplypressure, same can be caused to turn on the auxiliary supply pressureupon failure of the main supply pressure.

8 Claims, 3 Drawing Figures DOUBLE-HYDRAULIC ACTUATOR Hydraulicactuators, particularly those for controls of air and spacecraft, mustbe constructed redundantly for reasons of safety, that is, at least induplicate, so that in case one (asthe main) drive breaks down at leastone second (auxiliary) drive is able to automatically take over thefunction of the first one. However, this objective cannot be fulfilledmerely by mechanically coupling two so-called simple hydraulic actuatorswith the same output and by permitting same to operate a common outputmember.

Thus, for example in one known arrangement (German OffenlegungsschriftNo. l 426 480) wherein two tandem connected servo motors aresimultaneously hydraulically active and the control valves or controlvalve pistons thereof are coupled mechanically on both sides, additionalswitching devices are provided in the hydraulic control circuits so thata breakdown of an electric control circuit does not result in a blockingof the control valve pistons and thus result in a total breakdown of theentire actuator. Because of the mechanical coupling of the control valvepistons, it is however, not possible to avoid a total breakdown of thesystem upon a blocking of one valve member, as due to binding or seizingof the parts. In view of this, further measures are needed if one doesnot want to dispense all together with a mechanical coupling of thecontrol valve pistons, which dispensing is not possible in the case ofthis tandem actuator.

A further problem is that an actuator as above referred to requires,furthermore, an extremely exact adjustment of its coupled control valveswhich in turn requires a correspondingly great expenditure of effortduring manufacture thereof. If control valves of simple servo motors areto be coupled, and thus are designed substantially for use in so-calledsimple servohydraulics, then it is not possible to make symmetric thehydraulic functions of these control valves over their entire controlrange. Due to the different valve curves (stroke/flow) one must countboth on a considerable reduction of the regulating speeds and also onentirely asymmetric pressure relations in the operating cylinderchambers. It has been proven that for this reason operating pistonforces can occur which are partly opposed and thus causes the effectiveactuating force of the drive to be at least materially reduced.

The purpose of the invention is to produce a doublehydraulic actuatorwhich remains operative when one of its mechanically coupled controlvalves or control valve pistons becomes inoperative or when an energysupply breaks down.

A further purpose is to produce an actuator which no longer requires thehydraulic functions of the control valves to be symmetrical and whichwill not become inoperative. Upon the occurrence of a blocking, orseizing as has already been mentioned, for example, a sticking of acontrol valve piston.

Starting from a double-hydraulic actuator with at least two mechanicallycoupled servo motors in which also the piston rods of the controlvalves, with which a change-over device is associated, are coupled, itis suggested with respect thereto according to the invention, first toprovide between each control valve piston rod and a member whichtransmits its control signals a positive connection which limits thecontrol forces and, second, a change-over device which consistssubstantially of a switching element which selectively switches torender hydraulically active either the one or the other servo motor.

Thus such an actuator does not deal with a so-called truedouble-hydraulic, but instead, with two coupled servo motors of whichonly one works on a common output member while the other one moves alongin a pressureless manner as a standby. For this purpose, no furtherrigid connections are required directly between the servo motors asidefrom an operating piston rod coupling of the common type of constructionand a linkage which provides the coupling of the control valve pistonrods. It is understood that in this arrangement the aforementionedmembers which transmit control signals are structural parts of thecontrol linkage but the chosen positive connections between thosemembers and the control valve piston rods permit (namely during blockingof a control valve piston) an overriding of same. Thus, the second, oroperative, valve piston can then always be easily actuated. Because ofthis concept, only insignificantly greater control forces must beproduced than are normally required for operating of the control valvepistons.

In order to effect an automatic changing from the failing to theoperative servo motor, the aforementioned switching element is in activeengagement with at least one of the positive connections, advantageouslywith the one of the control valve piston rod of the servo motor whichserves as main drive.

A differential cylinder can be provided as switching element, whichcylinder on the piston side is loaded by the supply pressure of thefirst servo motor and on the piston rod side (annular chamber) by thesupply pressure of the second servo motor, whereby its piston rodactively engages a shut-off device for the pressure source whichsupplies the second servo motor. Aside from an automatic changing overduring a blocking of a control valve piston, this arrangement has theadvantage that the differential cylinder also in case of a breakdown ofa hydraulic energy circuit, preferably the main servo motor,automatically switches to a hydraulically active condition the second(auxiliary) servo motor which has been moving along on stand-by.

A further important advantage exists in that in such an actuator servomotors can also be used which are designed substantially only forso-called simple servohydraulics. Since during operation only one of theservo motors is hydraulically active, adjustment of its control valvesis no longer needed when the operation changes. The adjustment which iscommon in the manufacture of the (single) motors, as has already beenmentioned, is completely sufficient.

Thus the servo motors are also in case of operation breakdown or damageeasily and individually replaceable. At the same time this tends tosolve the problem of quickly installing double-hydraulic actuators whereneeded and to adjust same to the respective requirements (actuatingforces and the like). This problem exists particularly in carrying outprojects of the air and space flight especially, during the testingphase.

The invention and further developments of same are discussed more indetail hereinafter in connection with one exemplary embodiment, asillustrated in the drawing, in which:

FIG. 1 is a schematic view of a control in which only one mechanicallinkage is provided for transmitting of control signals to adouble-hydraulic actuator.

FIG. 2 is a perspective view of details of the connection between acontrol valve piston rod and a structural part of the linkage accordingto FIG. 1 and FIG. 2a is a partial view of a detail according to FIG. 2.

A control surface 1, for example an elevator of an aircraft, can bepivoted by two mechanically coupled servo motors 2 and 3 which areprovided therefore with operating cylinders having piston rods 4 and 5extending therethrough. The latter are axially fixedly connected by acoupling 6 of the usual type of construction, for example a connectingrod coupling; and they are radially movable, within limits, in order tocompensate for angle errors and the like. The servo motors 2 and 3 aresimple servo motors with an equal output and they actuate a commmonoutput member such as a linkage 7 which is connected to the controlsurface 1 and to new power piston rod 4 of the servo motor 2 which isused as a main drive. However, of the servo motors 2 or 3, which areconnected to a separate pressure source 8 or 9 only one is constantlyhydraulically active while the second one follows in readiness butwithout pressure thereon. This will be discussed more in detail below.

The control valves 10 and 11 of the servo motors 2 or 3 are coupled by alinkage in which, as in a parallelcrank mechanism, two two-arm levers 12or 13 are pivotally connected at one respective end of each thereof by apiston rod 14 and at the other respective ends of each thereof thelevers 12 or 13 are each hinged to one of the power piston rods 4 or 5.The piston rod 14 can for example be moved by a control stick which actson its left end 50. The control valves are of the type having so-calledby-pass mechanism (by-pass valve) which operates in response to pressureand act during reduction or failure of supply pressure to close anotherwise free passage for the pressure medium which passage normally isprovided between the chambers of the associated operating cylinder. Eachlever 12 and 13 is provided with a cap screw 15 which is arranged freelymovably in a not illustrated axially elongated slot in the associatedcontrol valve piston rod 16 or 17 (coupling part 18). The coupling parts18 are guided through the associated lever 12 or 13 which has thereforean opening 19 in the zone of its axis of rotation. This is illustratedin FIG. 2 only for the lever 12 with the understanding thatcorresponding parts are associated with the lever 13, comprise a mirrorimage thereof and hence need no express illustration. With respect tothe piston rod 14, it can, in view of an easier adjustment of thecoupled levers 12 and 13, be additionally equipped with the usual screwlocks or the like.

The connections for transmitting control forces between the levers l2,l3 and the associated control valve piston rods 16 or 17 are provided atthe outer ends of the coupling parts 18.- Details of these connections,which are the same in both control valve piston rods 16 and 17, areillustrated in FIG. 2 but only in connection with the control valvepiston rod 16 of the main servo motor 2. According to such figure theaforementioned cap screw 15 is used as a pivotal connector for two leverelements 20 which extend on both sides of the coupling part 18. Thelever elements 20 are at their outer ends 21 hinged pivotably, and atthe same time axially movably, to a further pivot means 22 which isfixedly arranged in the coupling part 18. Each lever element 20 has anelongated slot 54 (FIG. 2a) in which the pivot 22 abuts after a fullcontrol piston stroke in each control direction. To connect the pivotmeans 22 to the lever elements 20, a clamplike operating (as leaf)spring element 23 is used wherein each spring 23 holds the pivot 22 inthe elongated slot of the associated lever element 20 in a centralposition through movable sliding members 24. The outer ends 21 of thelever elements 20 are for this purpose constructed as bearing housingsfor these connecting elements (spring elements 23, sliding members 24).

As has already been mentioned, such a positive connection is providedbetween each lever 12 and 13 and the associated control valve piston rod16 or 17.

By this arrangement, the coupled levers l2 and 13 can be pivotedrelative to the associated power piston rod 4 or 5 even if one of thecontrol valve pistons (not illustrated) is blocked. The spring elements23 which oppose the respective control forces, in such case permits anaxial shifting of whichever of the lever elements 20 is associated withthe blocked piston and do so in each control direction. It is understoodthat this shifting is possible only if the normal control forces areexceeded and that only the spring elements which belong to these leverelements become effective.

A blocked control valve piston thus does not result in blocking thelinkage, instead, it is only necessary to produce greater control forcesto overcome and override the spring connectors. If the control valvepiston of the main servo motor 2 is blocked, then an automatic switch tothe second (auxiliary) servo motor 3 or its control valve 11 takes placewhereby this motor is switched hydraulically active and the faulty oneis depressurized. For this purpose a differential cylinder 25 isprovided which is loaded on its piston side 26 through a pressure line51 by the supply pressure 8 of the main servo motor 2 and on its pistonrod side 27 (annular chamber 28) through a pressure line 52 by thesupply pressure 9 of the auxiliary servo motor 3. The piston rod 29 ofthe differential cylinder 25 is thereby in active engagement with ashut-off device 31 which is connected into a supply line 53 whichconnects the auxiliary servo motor 3 to its pressure source 9. It can beseen (FIG. 1) that this shut-off device 31 does not influence theloading of the annular chamber 28 by the supply pressure of thelast-mentioned motor. Furthermore, a shut-off device 32 is connectedinto the pressure line 51 between the piston side 26 of the differentialcylinder 25 and the pressure source 8 which supplies the main servomotor 2, which shut-off device is operable in response to the springelements 23 of the connection of control valve piston rod 16 and lever12. This is indicated by the dash-dotted line 33. The shutoff valve 32can for example be a magnetic valve the circuit of which is closed oropened by a contact which is operated by the aforementioned springelements 23. Finally, a shut-off device 35 is connected into a supplyline 34 which connects the control valve 10 of the main servo motor 2 toits pressure source 8. Said shut-off valve is coupled through a linkage36 to the first shutoff device 31.

Only one of the two mechanically coupled shut-off devices 31 and 35, forexample spool valves, is open while the other one blocks or vice versa.If for example the third shut-off device 35 is open and thus the mainservo motor 2 is energized, the pressure medium supply to the auxiliaryservo motor 3 or its control valve 11 is blocked through the firstshut-off device 31. At the same time the second de-energized (and thusopen) shut-off device 32 assures that the piston 37 of the differentialcylinder is in its appropriate limit position, that is it is in theextended position which is illustrated in FIG. 1. This results fromequal supply pressure in that the force which acts onto the largerpiston surface from the supply pressure of the main servo motor 2 isgreater than the force (supply pressure auxiliary motor 31) which actson the smaller annular surface. Thus in this position also the piston 37keeps the first shut-off device 31 closed.

If the control valve piston of the main servo motor 2 is blocked, thepiston 37 of the differential cylinder 25 is automatically retracted,that is, it assumes the second limit position as soon as the spring 23of that control valve piston is overcome. This causes the auxiliaryservo motor 3 to become hydraulically energized and the main servo motor2 to become depressurized. More in detail, the yielding of therespective spring elements 23, which yielding occurs upon blocking of acontrol valve piston, causes a blocking of the second shut-off device 32after which only the annular chamber 28 of the differential cylinder 25is pressurized, namely, by the supply pressure of the auxiliary servomotor 3. In the second not illustrated limit position of the piston 37,the first shut off device 31 is now opened through the bent lever andthe third shut-off device 35 is, because of its mechanical coupling(linkage 36) to the first one, now closed. Thus, the power piston of themain servo motor 2 is prevented from getting into a locked condition.

Accordingly the auxiliary servo motor 3, whose control valve 11(particulary the piston thereof) has the same position as that of themain servo motor 2, can take over automatically the driving functionwhen the other is inoperative. To identify the blocked control valvepiston, one only needs to connect a signalling device to the secondshut-off device 32. Thus, if this device is not energized and greaterforces than usual must be produced for the control; then this indicatesa blocking of the control valve 11 of the auxiliary servo motor 3.

If the supply pressure of the main servo motor 2 is inoperative, theauxiliary servo motor 3 is switched automatically to a pressurizedcondition since again the pressure on the greater piston surface of thedifferential cylinder 25 is lacking and the piston 37, when it changesinto its second limit position, opens the first shut-off device 31through its piston rod 29 and the bent lever 30.

It is noted that the described arrangement is not limited to a doublehydraulic drive with two simple servo motors. In place of these motors,for example, motors can also be used in which three similar operatingcylinders are arranged parallel to one another and are provided with acommon valve unit. Such servo motors are usually used for controllingthe rotor blades of rotarywing aircraft. In such case it will beunderstood that then at least the aforedescribed linkage is constructedin triplicate.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

' 1. The combination in a redundant fluid actuator sys- 6 valve forcontrolling the movement of said first piston rods, each of said controlvalves having a second piston rod thereon, means for mechanicallycoupling each of said first piston rods together, a common controlmember for controlling both of said servomotors, said common controlmember including means for mechanically coupling said common controlmember to each of said first piston rods, means for preventing theoperation of one of said servomotors while permitting an operation ofthe other of said servomotors, and means for effecting a change-over inoperation from one of said servomotors to the other of said servomotors,said changeover means including means defining a resilient connectionbetween each of said second piston rods and said common control memberand switching means for effecting said change-over responsive to flexingof said resilient connection due to a force being applied thereto bysaid common control member which exceeds a predetermined limit causedwhen one of said second piston rods becomes blocked due to amalfunctioning thereof, and resilient connection also being sufficientlyrigid to effect a movement of said second piston rods by said commoncontrol member without a flexing of said resilient connection, saidmovement being permitted when said second piston rods are freelymovable.

2. The combination according to claim 1, wherein said common controlmember comprises a pair of levers and pivot connection means thereonpositioned adjacent one end and intermediate the ends of each of saidpair of levers, said second piston rods being connected to said commoncontrol member at said intermediate position whereby a movement of theother end of said control member in one direction will effect a movementof said second piston rods in said direction.

3. The combination according to claim 2, wherein said pivot connectionmeans adjacent said one end of said pair of levers is connected to saidmechanically coupled first piston rods of said servomotors.

4. The combination according to claim 1, including first and secondpressure source means for supplying pressurized gas to each of saidcontrol valves controlling said servomotors, respectively;

wherein said change-over valve means further includes first means foreffecting a maintaining of one of said control valves in fluidcommunication with said first pressure source means in response to thefluid pressure in said first pressure source means and for producing asignal in response to a drop in said gas pressure and wherein said meansfor preventing the operation of one of said servomotors while permittingan operation of the other of said servomotors includes second means foreffecting a shut off of the pressurized fluid supply to said one of saidcontrol valves in response to said signal and for simultaneouslyconnecting said second pressure source means to the other of saidcontrol valves.

5. The combination according to claim 1, including first and secondpressure source means for supplying pressurized fluid to each of saidcontrol valves controlling said servomotors, respectively;

wherein said change-over means further includes first means foreffecting a maintaining of one of said control valves in fluidcommunication with said first pressure source means in response to saidswitching means and for producing a signal in response to said flexingof said i'esilient connection beyond said predetermined limit andwherein said means for preventing the operation of one of saidservomotors while permitting an operation of the other of saidservomotors includes second means for effecting a shut off of thepressurized fluid supply to said one of said control valves in responseto said signal and for simultaneously connecting said second pressuresource means to the other of said control valves, and said first meansincluding a shut-off valve and differential pressure responsive actuatormeans.

6. The combination according to claim 1, wherein said resilientconnection means comprises a pair of lever elements pivotally secured atone end to each of said second piston rods and said control member, eachof said levers having means defining an elongated slotlike openingtherethrough, the longitudinal axis intersecting the axis of said pivotconnection to the respective one of said second piston rods, pin meanssecured to said second piston rod and received in said elongatedslot-like opening, said pin means being longitudinally slidable in saidelongated slot-like opening and leaf spring means for resilientlymaintaining said pin means in an approximate centrally located positionin said elongated slot-like opening whereby a relative movement betweensaid second piston rod and said levers will be resisted by said leafspring means.

7. The combination according to claim 6, including sliding membersengaging said pin means on opposite sides thereof, said leaf springmeans engaging oppositely facing sides of said sliding members toresiliently maintain said pin means in said central position.

8. The combination in a redundant fluid actuator system of at least twoseparate and separably operated servomotors, each of said servomotorshaving a first piston rod mounted for reciprocal movement and a controlvalve for controlling the movement of said first piston rods, each ofsaid control valves having a second piston rod thereon, means formechanically coupling each of said first piston rods together, a commoncontrol member for controlling both of said servomotors, said commoncontrol member including means for mechanically coupling said commoncontrol member to each of said first piston rods, means for preventingthe operation of one of said servomotors while permitting an operationof the other of said servomotors and means for effecting a change-overin operation from one of said servomotors to the other of saidservomotors, at least two pressure source means for supplyingpressurized fluid to each of said control valves controlling saidservomotors, respectively, said means for effecting said changeoverincluding means defining a resilient connection between each of saidsecond piston rods and said common control member and switching meansfor effecting said change-over responsive to flexing of said resilientconnection due to a force being applied thereto by said common controlmember which exceeds a predetermined limit caused when one of saidsecond piston rods becomes blocked due to a malfunctioning thereof, saidresilient connection also being sufficiently rigid to effect a movementof said second piston rods by said common control member without aflexing of said resilient connection, said movement being permitted whensaid second piston rods are freely movable, valve means for producing asignal in response to a flexing of said resilient connection and saidmeans for effecting a change-over further including pressuredifferential valve means connected in circuit with both of said pressuresource means and said valve means for effecting a shut off of thepressurized fluid supply to one of said control valves in response tosaid signal and for simultaneously connecting said second pressuresource means to the other of said control valves, said pressuredifferential valve means including a cylinder having a reciprocablepiston mounted therein and a rod connected to said piston and movabletherewith and wherein said means for preventing the operation of one ofsaid servomotors while permitting an operation of the other of saidservomotors includes first and second valves connected in circuit withsaid supply of pressurized fluid and each controlling the supply ofpressurized fluid to said control valves, said first and second valveseach having a control rod thereon which is mechanically coupled to theother and simultaneously operable in response to a movement of saidpiston to effect said shut off of said pressurized fluid supply to oneof said control valves and to simultaneously connect said secondpressure source means to the other of said control valves.

UNITED STATES PATENT oTTTcE QERTIHCATE 0F CURRECTWN Patent No. 3 878 764Dated April 1975 Inventor(s) Hans Zech It is certified that errorappears in the above-identified patent and that said Letters Patent arehereby corrected as shown below:

Column 6, line 21; change "andresilient" to -said resilient- 9 line 42;change gas" to fluid-.

line 44; delete "valve" line 50; change "gas to --fluid-.

gigncd and fizaizd this sixteenth Day of September-i975 [SEAL] A tres t:

RUTH C. MASON C. MARSHALL DANN Atlestl'ng Officer (ummisxiuner ofParentsand Trademarks

1. The combination in a redundant fluid actuator system of at least twoseparate and separably operated servomotors, each of said servomotorshaving a first piston rod mounted for reciprocal movement and a controlvalve for controlling the movement of said first piston rods, each ofsaid control valves having a second piston rod thereon, means formechanically coupling each of said first piston rods together, a commoncontrol member for controlling both of said servomotors, said commoncontrol member including means for mechanically coupling said commoncontrol member to each of said first piston rods, means for preventingthe operation of one of said servomotors while permitting an operationof the other of said servomotors, and means for effecting a change-overin operation from one of said servomotors to the other of saidservomotors, said change-over means including means defining a resilientconnection between each of said second piston rods and said commoncontrol member and switching means for effecting said change-overresponsive to flexing of said resilient connection due to a force beingapplied thereto by said common control member which exceeds apredetermined limit caused when one of said second piston rods becomesblocked due to a malfunctioning thereof, and resilient connectIon alsobeing sufficiently rigid to effect a movement of said second piston rodsby said common control member without a flexing of said resilientconnection, said movement being permitted when said second piston rodsare freely movable.
 1. The combination in a redundant fluid actuatorsystem of at least two separate and separably operated servomotors, eachof said servomotors having a first piston rod mounted for reciprocalmovement and a control valve for controlling the movement of said firstpiston rods, each of said control valves having a second piston rodthereon, means for mechanically coupling each of said first piston rodstogether, a common control member for controlling both of saidservomotors, said common control member including means for mechanicallycoupling said common control member to each of said first piston rods,means for preventing the operation of one of said servomotors whilepermitting an operation of the other of said servomotors, and means foreffecting a change-over in operation from one of said servomotors to theother of said servomotors, said change-over means including meansdefining a resilient connection between each of said second piston rodsand said common control member and switching means for effecting saidchange-over responsive to flexing of said resilient connection due to aforce being applied thereto by said common control member which exceedsa predetermined limit caused when one of said second piston rods becomesblocked due to a malfunctioning thereof, and resilient connectIon alsobeing sufficiently rigid to effect a movement of said second piston rodsby said common control member without a flexing of said resilientconnection, said movement being permitted when said second piston rodsare freely movable.
 2. The combination according to claim 1, whereinsaid common control member comprises a pair of levers and pivotconnection means thereon positioned adjacent one end and intermediatethe ends of each of said pair of levers, said second piston rods beingconnected to said common control member at said intermediate positionwhereby a movement of the other end of said control member in onedirection will effect a movement of said second piston rods in saiddirection.
 3. The combination according to claim 2, wherein said pivotconnection means adjacent said one end of said pair of levers isconnected to said mechanically coupled first piston rods of saidservomotors.
 4. The combination according to claim 1, including firstand second pressure source means for supplying pressurized gas to eachof said control valves controlling said servomotors, respectively;wherein said change-over valve means further includes first means foreffecting a maintaining of one of said control valves in fluidcommunication with said first pressure source means in response to thefluid pressure in said first pressure source means and for producing asignal in response to a drop in said gas pressure and wherein said meansfor preventing the operation of one of said servomotors while permittingan operation of the other of said servomotors includes second means foreffecting a shut off of the pressurized fluid supply to said one of saidcontrol valves in response to said signal and for simultaneouslyconnecting said second pressure source means to the other of saidcontrol valves.
 5. The combination according to claim 1, including firstand second pressure source means for supplying pressurized fluid to eachof said control valves controlling said servomotors, respectively;wherein said change-over means further includes first means foreffecting a maintaining of one of said control valves in fluidcommunication with said first pressure source means in response to saidswitching means and for producing a signal in response to said flexingof said resilient connection beyond said predetermined limit and whereinsaid means for preventing the operation of one of said servomotors whilepermitting an operation of the other of said servomotors includes secondmeans for effecting a shut off of the pressurized fluid supply to saidone of said control valves in response to said signal and forsimultaneously connecting said second pressure source means to the otherof said control valves, and said first means including a shut-off valveand differential pressure responsive actuator means.
 6. The combinationaccording to claim 1, wherein said resilient connection means comprisesa pair of lever elements pivotally secured at one end to each of saidsecond piston rods and said control member, each of said levers havingmeans defining an elongated slot-like opening therethrough, thelongitudinal axis intersecting the axis of said pivot connection to therespective one of said second piston rods, pin means secured to saidsecond piston rod and received in said elongated slot-like opening, saidpin means being longitudinally slidable in said elongated slot-likeopening and leaf spring means for resiliently maintaining said pin meansin an approximate centrally located position in said elongated slot-likeopening whereby a relative movement between said second piston rod andsaid levers will be resisted by said leaf spring means.
 7. Thecombination according to claim 6, including sliding members engagingsaid pin means on opposite sides thereof, said leaf spring meansengaging oppositely facing sides of said sliding members to resilientlymaintain said pin means in said central position.